1. Field of the Invention
The invention relates to medical diagnostic sensor packaging. More particularly, the invention relates to a system and method for sealing a one-time use medical diagnostic sensor package.
2. Background Art
In the area of blood testing on patients using various sensing technologies, it is well known that the sensors must be properly calibrated if the measurement is to be performed with the desired degree of accuracy. Recent developments in clinical diagnostics have led to the development of unitized testing systems where a sensor is packaged into a device that is used for a single panel of tests and then discarded. These devices are typically used in conjunction with a reader that is able to interact with the device. Interactions include extracting a signal from each sensor, and optionally controlling the motion of fluids within the device, e.g., positioning of the sample and a calibrant fluid with respect to the sensor. A detailed description of one such system, referred to herein as the i-STAT™ system, is found in U.S. Pat. No. 5,096,669 (the '669 patent), which is jointly owned and incorporated herein by reference.
A key feature of these sensing systems is that the devices are manufactured and shipped to customers on a regular basis. The time between the manufacture and use of the device, however, can be several months. As a result, devices are manufactured with labeling that indicates the available shelf-life under a given set of conditions, e.g., six months with refrigeration, and two weeks at room temperature, among other combinations of storage conditions.
There are several reasons why shelf life of a given sensor may be limited, including, but not limited to sensor stability and calibrant fluid stability. With respect to the calibrant fluid, it is important that the concentration of the calibrant analyte (e.g., potassium ion concentration, partial pressure of carbon dioxide, among others), does not change during storage. One solution to this problem is to store calibrant fluids in a sealed glass vessel, or ampoule. In a sealed vessel, the walls of the vessel do not permit gas or liquid exchange. However, when devices are designed for convenient use, e.g., in the bedside or point of care testing environment, it may not be practical to use a glass storage vessel. The impracticalities can relate to fragility, and issues of packaging a glass element into a test housing, e.g., a single-use test cartridge. As a result, foil pouches with plastic layers have been used to affect the seal. For example, the '669 patent discloses calibrant packs that are made with plastic-lined foil having a perimeter seal. Specifically, two portions of plastic-lined foil with plastic faces abutted are sealed together to form an enclosure containing a liquid phase and a gas phase. Here, the perimeter seal is formed by applying sufficient heat to melt the plastic and sufficient pressure to form a contiguous plastic perimeter seal. Within the enclosure (or pouch), the liquid phase comprises a calibrant fluid, e.g., a buffered aqueous mixture containing known concentrations of the analytes to be tested, including, for example, potassium, sodium, glucose, and lactate, among others. The gas phase in the pouch can be, for example, air or a desired gas composition, e.g., 5% carbon dioxide, 20% oxygen and 75% nitrogen. The gas phase, or the dissolved gases in the liquid phase, can also act as a calibrant, e.g., for blood gas sensing of the partial pressures of oxygen and carbon dioxide, pO2 and pCO2 respectively.
With regard to the construction of the pouch, the choice of foil, e.g., ˜40 μm aluminum roll, is determined by its barrier properties, i.e., the resistance to transport gases, vapors and liquids. Foils are also preferably selected to minimize pin-holes. Various optical inspection means are well known in the art for identifying pin-hole failures. The plastic layer serves as a means for providing a seal and also protecting the fluid from direct contact with the metal foil, which can cause degradation of one or more of the calibrant fluid components.
While the foil is generally an effective barrier, various gases, e.g., oxygen, carbon dioxide and water vapor are soluble in plastics to different degrees and also can permeate the plastic matrix at a given rate. This rate will be a function of temperature and pressure, the chemical composition of the plastic, the solvent from which it is cast, and the density of the cast material.
Where a specific gas is used for calibration purposes, e.g., a known partial pressure of carbon dioxide (pCO2) to calibrate a pCO2 sensor, it is preferable for the seal to have a low permeability and solubility for pCO2. The dimension of the housing into which the fluid-containing pouch is to be packaged, however, may place restrictions on the seal dimensions.
Packaging of a pouch into a small plastic housing is shown in the '669 patent. Here the pouch sits in a plastic base with a barb structure capable of piercing the pouch. The pouch is held in place by double-sided adhesive tape attached to a plastic cover. The plastic cover has a flexible paddle directly above and aligned with the pouch. When a force is applied to the paddle, it presses the pouch against the barb, rupturing the pouch and releasing the calibrant fluid to flow through a conduit and into contact with an arrays of sensors.
A further consideration, where possible, is to minimize gas exchange across the seal by minimizing the driving force, i.e., the difference in pressure and concentration of the analyte on either side of the seal. A reduced temperature can also reduce gas exchange, however this approach must be used judiciously, as freezing an aqueous fluid within a pouch may lead to undesirable effects such as seal rupture. As a consequence, refrigeration is a useful compromise.
Regarding other art, U.S. Pat. No. 6,178,832 (hereinafter the '832 patent) describes a self-contained reagent chamber with fluids including tonometered calibrants where the chamber wall includes multiple layers of materials and where at least one layer is a thin, flexible glass material. The walls are extended to form a filler neck sealed by heat and pressure along a sealing line below a filler line, so that no bubbles are trapped in the reagent chamber.
U.S. Published Patent Application No. 20060013744 discloses a flexible container for a reference gas, for use in performing calibration or quality control of an apparatus for determining a gas parameter in a physiological liquid, such as blood. The flexible container is adapted to hold the reference gas at or close to ambient pressure.
U.S. Published Patent Application No. 20060183216 discloses a container for a liquid reagent, wherein the container has an outer wall and an internal piercing member. Such a container is configured to store the liquid for periods between 6 to 18 months with minimal loss of the liquid inside, other than if the container is ruptured. The container is preferably adapted for use with a micro-fluidic device.
U.S. Published Patent Application No. 20040222091 discloses a diagnostic device incorporating electrode modules and fluidics for performing chemical analyses. The device consists of a plastic card-like body with fluidic conduits and a sealed fluid reservoir contained in a foil-lined cavity. The reservoir holds a calibrant fluid that is used to calibrate the electrodes.
Conventional fluid-containing pouches of the type described in the '669 patent have proved commercially successful for calibrating blood testing sensors where the pouches have an extended shelf-life with refrigeration. However, the need exists for improved fluid-containing pouches that have an extended shelf-life without refrigeration, such that their contents remain substantially unaltered with extended room temperature storage.